Abstract
Desertification is a serious threat to the ecological environment and social economy in our world and there is a pressing need to develop a reasonable and reproducible method to assess it at different scales. In this paper, the Ordos Plateau in China was selected as the research region and a quantitative method for desertification assessment was developed by using Landsat MSS and TM/ETM+ data on a regional scale. In this method, NDVI, MSDI and land surface albedo were selected as assessment indicators of desertification to represent land surface conditions from vegetation biomass, landscape pattern and micrometeorology. Based on considering the effects of vegetation type and time of images acquired on assessment indictors, assessing rule sets were built and a decision tree approach was used to assess desertification of Ordos Plateau in 1980, 1990 and 2000. The average overall accuracy of three periods was higher than 90%. The results showed that although some local places of Ordos Plateau experienced an expanding trend of desertification, the trend of desertification of Ordos Plateau was an overall decrease in from 1980 to 2000. By analyzing the causes of desertification processes, it was found that climate change could benefit for the reversion of desertification from 1980 to 1990 at a regional scale and human activities might explain the expansion of desertification in this period; however human conservation activities were the main driving factor that induced the reversion of desertification from 1990 to 2000.
Highlights
Desertification is recognized as one of the most serious social-economic-environmental issues in arid, semi-arid and dry sub-humid areas, and in more than one hundred countries, about one billion of the six billion world population are affected by desertification [1,2]
These changes of land surface conditions make the spectral characteristics of desertification land vary greatly to different degrees, which can be captured by satellite sensors and this may be fundamental for quantitatively assessing desertification by means of the indices derived from satellite images [14]
According to the rule sets, the change of Normalized Difference Vegetation Index (NDVI) and albedo had a linear relationship with the change of desertification grade, which showed that the reversion of desertification was characterized by the increasing of NDVI or the decreasing of albedo
Summary
Desertification is recognized as one of the most serious social-economic-environmental issues in arid, semi-arid and dry sub-humid areas, and in more than one hundred countries, about one billion of the six billion world population are affected by desertification [1,2]. The definition of desertification put forward by United Nations Convention to Combat Desertification [3] in 1994, that “desertification is land degradation in arid, semiarid and dry sub-humid areas resulting from various factors, including climatic variations and human activities” has been widely accepted, there is no consensus concerning assessment systems and methodology on the proper way to assess it and this may result in desertification being measured by different researchers using different assessment methods that cannot be compared for the same region [4]. As a consequence of climatic variations and human activities that affect desertification, vegetation conditions in arid, semiarid and dry sub-humid environment have greatly changed, and these changes include the reduction of vegetation cover, density and biomass, and are characterized by structural configurations of vegetation types and landscape patterns [5,9,10]. These changes of land surface conditions make the spectral characteristics of desertification land vary greatly to different degrees, which can be captured by satellite sensors and this may be fundamental for quantitatively assessing desertification by means of the indices derived from satellite images [14]
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